Reactive metal nanoparticles may be used to accelerate or catalyze the combustion of fuels and/or explosives. For instance, reactive nanoparticles can be added to solid rocket propellants or to high-energy explosives to increase the amount of heat generated and energy released during combustion of the nanoparticle containing fuel and/or explosive. Typically, a small quantity of reactive nanoparticles can be mixed with a fuel, explosive, and/or other major component to form an energy releasing composition. The nanoparticles provide an enhanced reaction rate to the composition due to the enormous amounts of heat generated at a local level as the nanoparticles react with oxygen during combustion of the composition. The energy release provided by the nanoparticles increases the reaction rate of the composition, providing increased energy or heat to the reaction.
Typical reactive nanoparticles used to form reactive compositions include aluminum and molybdenum. Other reactive metal particles, such as zirconium, silicon, phosphorous, boron, sodium, magnesium, titanium, and potassium may also be used to form nanoparticles for use with reactive compositions.
The reactivity of nanoparticles used in the formation of reactive compositions poses problems in the production of such compositions because the nanoparticles are typically instable or reactive to the atmosphere. To counter the innate reactivity of the nanoparticles, the nanoparticles used to produce reactive compositions are typically coated with metal oxide surface coatings, which protect the nanoparticle from reaction with air. Metal oxides can be formed on a nanoparticle when the nanoparticle is exposed to oxygen. For example, nanoparticles may be exposed to an oxygen environment during manufacture to create a metal oxide passivation layer on the nanoparticles. Metal oxides can also form on nanoparticles from exposure to air or oxygen such as occurs over time during long-term storage of the nanoparticles.
Protective metal oxide surface coatings may consume up to 50 percent or more of the mass of a nanoparticle. The metal oxide portion of a metal oxide passivated nanoparticle is non-reactive and does not contribute to an increased energy or heat release upon combustion of the reactive composition employing the reactive nanoparticles. The overall energy per unit mass gain obtained by adding reactive nanoparticles to a reactive composition is thereby reduced by the mass of the metal oxide coating. The presence of the unreactive metal oxide in a reactive composition can also slow the rate of reaction of the composition or hinder the efficiency of a reaction of the composition.
Another drawback to the use of metal oxide coated reactive nanoparticles in reactive compositions is the fact that the oxide-coated nanoparticles often fail to disperse evenly in a reactive composition. For example, the hydrophobic nature of organic fuels such as gasoline and plastic explosives tends to encourage uneven distribution of oxide-coated nanoparticles in such fuels. The uneven distribution of reactive nanoparticles throughout the fuel can result in an uneven burn rate within the mass of fuel, with “hot spots” that burn faster due to locally higher concentrations of nanoparticles and “cold spots” that burn slower due to locally lower concentrations of nanoparticles.
Alternatives to metal oxide layers have been proposed as passivation coatings for reactive nanoparticles used with reactive compositions. For instance, reactive nanoparticles have been coated with polymers. Polymer coatings, however, can coat the particles unevenly, leaving pores and gaps through which oxygen can reach the reactive metal and form oxides on the surface of the coated nanoparticle. In addition, the polymer chains must be broken during reaction, which consumes energy from the reaction, and longer polymer chains require a greater amount of energy disengage from the nanoparticle. Thus, the thermal stability of the polymer chains becomes detrimental to the reaction because too much energy is consumed breaking the polymer chains to reach the nanoparticles.
Organic molecular monolayers have also been studied as passivation coatings for reactive nanoparticles. Such layers provide good stability against oxidation, impact sensitivity, and particle agglomeration. Organic molecular monlayer covered nanoparticles also retain good dispersion properties in fuel, propellant, and explosive composites.
Other coatings that have been proposed as passivation coatings for reactive metal nanoparticles include ceramics and alloys, such as inorganic salts or metals. For example, pyrolytic methods for producing reactive metal nanoparticles that simultaneously deposit a thin layer of sodium chloride on their surface to protect the particles from the atmosphere have been reported. In other instances, a thin film of gold has been used to protect magnetic iron particles from oxidation.
Although alternative reactive nanoparticle passivation coatings may be used in place of oxide coatings, these coatings can still incur some of the same downfalls as the oxide coatings. Surface oxides, ceramic coatings, inorganic coatings, organic polymer coatings, and alloy coatings can be extremely resistant to corrosion and/or combustion, thereby presenting a barrier to initiation of a reaction with the reactive portion of the nanoparticle. These alternative coatings also reduce the mass to energy ratio of the nanoparticles because the coatings are unreactive. Combustion rates with the alternative coatings can also be reduced due to the thermal stability of the coatings or resistance of the coatings to reaction.
Therefore, improved reactive nanoparticle passivation coatings that alleviate at least some of the problems of the currently available reactive nanoparticle passivation coatings are desirable.